This invention relates to melting of glass and the like, and in particular, to improvements in the use of submerged combustion in the melting process.
The use of submerged combustion to melt glass has been proposed in several patents including U.S. Pat. Nos. 3,170,781; 3,224,855; 3,237,929; 3,260,587; 3,606,825; 3,627,504; 3,738,792; and 3,764,287. In submerged combustion, the combustion gases are injected beneath the surface of a molten pool and are permitted to percolate upwardly through the melt. An advantage of such an approach is that the material being heated is in intimate contact with the combustion gases and the thermal energy released therefrom, thereby yielding very favorable heat exchange rates. Another advantage is that the injection of the gases into the melt produces a high degree of stirring which can be beneficial in some melting processes.
However, a significant drawback of submerged combustion is that the injection of large volumes of gas into the melt is in some cases difficult to reverse. In other words the melt, in particular, glass, can sometimes be converted into a foamy condition, and subsequently collapsing the foam can be difficult. Then tendency of submerged combustion to increase the gaseous content of the melt has been a deterrent for the acceptance of submerged combustion in the glassmaking industry, since one of the objectives of the glass melting process is to eliminate as much as possible bubbles and other gaseous inclusions from the molten glass. Proposals for using submerged combustion to melt glass have generally limited its application to the initial stages of the melting process to avoid increasing gaseous inclusions during the later stages of the melting and refining process.